The most important transport metabolite for stored energy in many plants, for example potatoes, is sucrose. In other species, other oligosaccharides can serve this role. In Japanese artichokes for example it is stachyose.
The central position of the oligosaccharide transport in the energy content of the plant has already been shown in transgenic plants, in which by expression of an invertase, the sucrose is split into the monosaccharides, leading to considerable changes in its habit (EP 442 592). Because of the significance of sucrose in the formation of storage materials, numerous experiments have been carried out into investigating the biosynthesis or the metabolism of disaccharides. From DE 42 13 444, it is known that the improvement of the storage properties of the harvested parts can be achieved in transgenic potatoes, in which through expression of an apoplastic invertase, the transfer of energy rich compounds to the heterotrophic parts of growing shoots is inhibited.
In spite of much effort, the mechanism for distributing storage materials, such as oligosaccharides in plants has not been clarified and, in order to influence it, it is not yet known, how the sucrose formed in the leaves following photosynthesis, reaches the transport channels of the phloem of the plant and how it is taken up from the storage organs, e.g. the tubers of potato plants or seeds. On isolated plasma membranes of cells of leaf tissue of sugar beet (Beta vulgaris) it has been demonstrated that the transport of sucrose through the membrane can be induced by providing an artificial pH gradient and can be intensified by providing an electrochemical potential (Lemoine & Delrot (1989) FEBS letters 249: 129-133). The membrane passage of sucrose follows a Michaelis-Menten kinetic, in which the k.sub.m value of the sucrose transport is around 1 mM (Slone & Buckbout, 1991, Planta 183: 484-589). This form of kinetic indicates the involvement of transporter protein. Experiments on plasma membranes of sugar beet, Ricinus communis and Cyclamen persicum has shown that the sucrose transport is concerned with a cotransport of protons (Buckhout, 1989, Planta 178: 393-399; Williams et al., 1990, Planta 182: 540-545; Grimm et al., 1990, Planta 182: 480-485). The stoichiometry of the cotransport is 1:1 (Bush, 1990, Plant Physiol 93: 1590-1596). Mechanisms have also been proposed however, for transport of the sucrose through the plasmodium of the plant cells (Robards & Lucas, 1990, Ann Rev Plant Physiol 41: 369-419). In spite of the knowledge of the existence of an active transport system, that allows the cells to deliver sucrose to the transport channels, a protein with these kind of properties is not yet known. In N-ethylmaleinimide staining of sugar beet plasma membrane in the presence and absence of sucrose Gallet et al. (1989, Biochem Biophys Acta 978: 56-64) obtained information that a protein of size 42 kDa can interact with sucrose. Antibodies against a fraction of plasma membrane protein of this size range can inhibit the sucrose transport through plasma membranes (Lemoine et al., 1989, Bichem Biophys Acta 978: 65-71). In contrast, information has been obtained (Ripp et al., 1988, Plant Physiol 88: 1435-1445) by the photoaffinity marking of soyabean protein with the sucrose analogue, desoxyazidohydroxybenzamidosucrose, on the participation of a 62 kDa protein in the transport of sucrose through membranes. An amino acid sequence of a sucrose transporter is not known.